1. Field of the Invention
The present invention relates to new phenol resins containing isopropenylphenol oligomer as partial or whole phenol component of the resin and an aldehyde. The invention further relates to varnishes containing the new resin and to laminates having superior mechanical and electrical properties and impregnated with the resins.
2. Description of the Prior Art
Present-day laminates are manufactured by impregnating a base such as paper or the like with a phenol resin prepared by reacting such substances as phenols, cresols or alkylphenols with formaldehyde and curing the resin. These laminates are widely used as decorative panels or as heat-resisting or electrically insulating materials. In recent years, laminates possessing high mechanical and electrical properties are in particular demand because of the vastly emerging development in telecommunications and electronic instruments. Especially, the commonly used copper-clad laminates are required not only to possess a high degree of electrically insulating ability but also to be satisfactory in many other properties such as punching quality, surface resistivity, volume resistivity, soldering heat resistance, dimensional stability, dielectric constant, dielectric tangent, and water-absorption. For example, a phenol resin containing as the phenol component thereof an alkylphenol such as cresol, propylphenol, butylphenol, octylphenol or nonlyphenol is widely used for improving punching quality. A drying oil such as linseed oil, tung oil or cashewnut oil is used as a plasticizer in this case. However, the use of such plasticizer for improving punching quality has a demerit in that other useful and/or necessary physical properties of the resin are thereby worsened, such as for example insulation resistance, especially insulation resistance after boiling.